Troubleshooting Solar String Underperformance and Mismatch

String-level underperformance is a common issue in commercial and utility-scale solar arrays. When one string consistently produces less than its neighbors, the cause can range from simple shading to complex cell degradation. String-level monitoring — when available — makes it possible to identify and diagnose these issues remotely, often without a site visit.

What Is String Mismatch?

In a solar array, strings of modules connected in series feed into a common inverter MPPT input. When all strings perform equally, the inverter operates at the optimal power point. When one string underperforms — due to shading, degradation, or a connection issue — it can pull down the performance of the entire MPPT input, reducing total system output. This is known as mismatch loss.

Common Causes of String Underperformance

• Partial shading from vegetation growth, new construction, or accumulated soiling on part of the string
• A cracked or degraded module within the string reducing the string's overall voltage and current
• A failed bypass diode causing a sub-string of cells to be bypassed, reducing the module's voltage output
• A loose or corroded DC connector adding series resistance and reducing current
• A blown string fuse in the combiner box (complete loss of the string)
• PID (potential-induced degradation) affecting modules in a specific section of the array
• Uneven soiling — dust accumulation on one section of the array reducing output
• Thermal mismatch from poor ventilation under a specific section of the array

Using String-Level Monitoring Data

If your monitoring platform provides string-level data, you can compare the current and voltage of each string on the same MPPT input under similar irradiance conditions. Look for strings that consistently produce 5-15% less than the average of their peers.

1. 1Export string-level production data for a clear-sky day with stable irradiance. Compare strings on the same MPPT input — strings on different inputs may have different orientations or inverter settings.
2. 2Calculate the percentage difference between each string and the average of all strings on that input. Flag any string that is more than 5% below average.
3. 3Check if the underperformance is consistent throughout the day or varies with sun angle. Shading causes time-dependent underperformance, while degradation and connection issues cause consistent underperformance.
4. 4Compare string voltage as well as current. Low voltage suggests a degraded module or failed bypass diode. Low current suggests shading, soiling, or a series resistance issue.
5. 5Review historical data to determine when the underperformance started. A sudden drop suggests an event (damage, fuse failure), while gradual decline suggests degradation.

Field Verification

Once you've identified an underperforming string from monitoring data, field verification helps confirm the cause.

• Visually inspect the string for shading, soiling, or physical damage.
• Perform an IV curve trace on the string to identify whether the issue is current-related (shading, soiling) or voltage-related (degradation, bypass diode failure).
• Use a thermal camera to check for hotspots on individual modules.
• Check DC connections at the combiner box for tightness and corrosion.
• Test string fuses for continuity.

When to Call for Remote Support

If your monitoring platform provides string-level data but you're not sure how to interpret it, or if you've identified underperforming strings but need help determining the root cause, remote support can help. By analyzing your string-level data alongside weather data and system configuration, a specialist can narrow down the likely cause and recommend targeted field actions.